This invention pertains to a method of preparing a magnetostrictive coating on a substrate where the coating is a composite of discrete magnetostrictive RE-Fe2 particles in a strengthening metal matrix. More specifically this invention pertains to a high velocity, high kinetic energy spray process for applying a particulate mixture of magnetostrictive compound and matrix metal on a substrate as a strong, low porosity magnetostrictive composite. The subject method is particularly useful for forming such coatings on round shafts, such as automotive steering columns, to serve in a torque sensing system.
Magnetostriction occurs when a material develops significant strain on exposure to a magnetic field. At room temperature sample dimensions can change by as much as fractions of a percent. Conversely, the straining of a magnetostrictive material changes its magnetization state.
Magnetostrictive materials have been used with electromagnetic actuators to form transducers which serve as, for example, ultrasonic generators or fine control valves for metering of fluids. In these applications, variation of an imposed magnetic field produces proportional strains in the magnetostrictive material to produce a mechanical output. Conversely, a magnetostrictive material can be used as a torque sensor in the form of a magnetostrictive ring mounted on a shaft such as an automobile steering shaft. Torque in such a shaft would strain the magnetostrictive ring, giving rise to a detectable change in the ring""s magnetization.
Maximizing device performance naturally suggests using materials having large saturation magnetostriction, xcexs, which is a dimensionless measure of the field-induced strain, frequently expressed in parts per million (ppm). High values of xcexs are found in rare earth-iron compounds such as the terbium-iron compound, TbFe2, where xcexs equals 1750 ppm for a polycrystalline sample. Unfortunately, the rare earth-iron compounds are very brittle materials having little tensile strength, an unfavorable property for automotive applications requiring good mechanical properties. On the other hand, stronger and tougher materials such as steels have very limited magnetostriction. T250 maraging steel, which is currently being evaluated in torque sensors, has a xcexs of onlyxe2x88x9230 ppm.
Furthermore, it is not easy to attach a T250 maraging steel ring (or other rings of magnetostrictive material) to a steering shaft in sufficiently intimate contact to make a suitably sensitive automotive torque sensor. Hardened rings of magnetostrictive T250 maraging steel are machined from bar or tube stock and attached to hardened nitronic steel shaft by one of two procedures. A tapered ring is press fitted onto a shaft having a matching taper, or a ring is brazed onto a hollow shaft followed by ballizing (pressing an oversized ball bearing down the inside of the shaft).
Strong, durable and machinable composite compositions displaying high magnetostrictive strains have been described in U.S. Pat. Nos. 5,985,049 and 5,993,565 to Pinkerton, Herbst, Capehart, Murphy and Brewer, both patents entitled xe2x80x9cMagnetostrictive Compositesxe2x80x9d and assigned to the assignee of this invention. These composites include magnetostrictive materials of the rare earth-iron compound type, RE-Fe2, where RE is one or more of the rare earth elements including yttrium, preferably samarium or terbium. While these rare earth-iron compounds display significant magnetostriction they cannot withstand machining or even normal handling without breaking. The patents teach compositions and methods by which particles of the rare earth-iron compounds are mixed with particles of a strong, suitably malleable metal such as aluminum, copper, iron, magnesium or nickel and the mixture suitably hot pressed to make a strong and useful magnetostrictive body. In the above patented practices, the hot pressing is controlled so as to form a nearly fully densified composite body without degrading the magnetostriction of the rare earth-iron compound particles.
Thus, the compositions and methods of the ""049 and ""565 patents enable the manufacture of strong machinable composite bodies for applications in which magnetostriction is a required or desired property. One such application for magnetostrictive materials is in torque sensors, for example, in electronically controlled, automotive power steering systems. Such sensors are described in U.S. Pat. No. 5,907,105 to Pinkerton, Herbst, Capehart, Perry and Meyer, entitled xe2x80x9cMagnetostrictive Torque Sensor utilizing RE-Fe2xe2x80x94Based Composite Materialsxe2x80x9d and assigned to the assignee of this invention. In that application, circumferentially magnetized, hot pressed ReFe2xe2x80x94matrix metal composite bodies in the form of rings are used in or on steering column shafts as part of a sensing system to detect the driver applied torque to the shaft.
Like the application of magnetostrictive T250 steel materials, the hot pressing and machining of a composite ring and the fitting of the ring onto the shaft, or inserting the ring as part of a steel or aluminum shaft, also presents manufacturing complexity. These manufacturing issues raise the question as to whether REFe2 magnetostrictive material could be applied in a simpler method to a substrate such as a steering column so as to retain the useful responses to torque changes of the above described hot pressed composite bodies.
This invention provides a method of forming a magnetostrictive composite coating of discrete rare earth-iron compound particles and metal matrix material on a desired substrate. In accordance with a preferred embodiment of the invention, the coating is applied to a suitable steel or aluminum automobile steering shaft to serve as a portion of a torque sensor device for determining angular position of the shaft in an electronically controlled power steering system.
A mixture of magnetostrictive ReFe2 powder and metal matrix material powder is sprayed onto a suitable substrate by a relatively low temperature supersonic velocity spray process sometimes called kinetic spraying. Kinetic spray processes are described in U.S. Pat. No. 6,139,913 and 5,302,414. The mixture is transported from a powder reservoir in a relatively low volume, high pressure stream of unheated gas and introduced into a larger volume, high pressure stream of heated carrier spray gas. The combined stream of gas and suspended powder undergoes adiabatic expansion through a suitable converging-diverging nozzle, such as a de Laval nozzle. During passage through the converging-diverging nozzle the stream achieves a very high velocity, a supersonic velocity, with particles accelerating due to drag effects with the high velocity gas. The carrier spray gas is heated to increase its velocity in the diverging portion of the nozzle. These high kinetic energy particles are directed against a desired substrate such as a steering column. The spray nozzle is moved in a suitable pattern over or around the substrate to accumulate a coating pattern of desired thickness. The substrate is not normally preheated but it may experience some temperature increase from the high energy impact of the sprayed particles.
As the high velocity particles impact the substrate they form a well adhered composite coating. The softer matrix metal particles deform to envelop individual REFe2 particles. The character and chemical identity of the rare earth-iron compound particles are unchanged but they are enclosed in a mechanically formed matrix of the metal composition to provide magnetostrictive properties to the low porosity coating. The proportions of REFe2 particles and matrix metal particles may be widely varied depending upon the desired balance of magnetostriction and mechanical properties sought in the composite coating. Usually it will be preferred to maintain the proportions of the two types of particles within the range of twenty to eighty percent of REFe2 particles and the balance the metal particles.
Thus, for example, a circumferential annular band of the composite material can be formed on a steering shaft and then magnetized circumferentially for use in a magnetostrictive torque sensor as illustrated, e.g., in FIGS. 1A and 1B of the above identified ""105 patent.
The rare earth-iron compounds may contain any of the rare earth elements or yttrium as described in the above referenced patents. It is generally preferred to use samarium or mixtures of samarium and dysprosium, or mixtures of terbium and dysprosium because of their availability and the high magnetostriction that they provide in their compounds with iron. The matrix metal can also be any of the metals (or their suitably ductile alloys) used or described in the referenced patents. For example aluminum, copper, iron, molybdenum, stainless steel and tantalum powders have been used as matrix metals and brass and various steels have been used as substrates in the practice of this invention. Since the rare earth compounds are readily oxidized, the powder carrier gas and the spray gas may be non-oxidizing gases such as, e.g., nitrogen or helium. The main carrier gas is heated to increase the velocity of the gas stream as it flows through the spray nozzle. A temperature is chosen for this purpose, usually less than about 1200xc2x0 F., at which the particles are not softened nor the composition or crystalline form of the REFe2 compound altered.
It is apparent that the mixture being sprayed contains particles of different physical characteristics that may affect their tendency to adhere to a substrate. Moreover, the relative shapes of the spray pattern and the substrate can affect the yield of sprayed particles that adhere to the substrate. Depending upon actual experience with a specific REFe2/metal particle mixture and substrate shape it may be necessary to adjust the proportions of the constituents to achieve a specified magnetostrictive composite composition.
The kinetic spray process used in the practice of this invention provides a relatively simple way to form low porosity, magnetostrictive composite coatings on a substrate that does not require hot pressing to consolidate the composite. Furthermore, it has been discovered that, in many instances, initially magnetically soft magnetostrictive particles become magnetically hard after their high velocity impact on the substrate. Magnetic coercivities of greater than 1000 Oersteds have been observed in the composite coatings.
Other objects and advantages of the invention will become apparent from a description of preferred embodiments.